1. Field of the Invention
The present invention relates to a spatial light modulator used in an optical computing device and a projection type display and also to a neural network circuit executing input/output operations similar to those of nervous system, e.g., pattern recognition, associative memory and/or parallel computation.
2. Description of the Related Art
The spatial light modulator is an important element for achieving an optical operation, e.g., optical logical operation or optical neurocomputing. In addition, the parallelism of light matches a neural network executing operations, with the use of parallel dynamics. Thus, various arrangements for optical neurocomputing have been provided. In particular, as an arrangement realizing a hierarchical neural network circuit in the form of hardware having a learning function, Ishikawa et al have proposed an optical system employing a reflective spatial light modulator (i.e., microchannel modulator) (Masatoshi Ishikawa et al., Applied Optics, 28 (2), 1989, pages 291-301).
In addition, one of important roles of neurocomputing is a nonlinear input/output characteristic operation or a threshold operation. Therefore, a transmissive spatial light modulator executing such operation is also required for materializing the neural network into a practical hardware form. However, there has been known no actual optical neurocomputer employing a transmissive spatial light modulator having the function of executing the threshold operation. Currently, the threshold operation of a reported optical neurocomputer depends on an electronic circuit or electronic computer.
An optically writing type spatial light modulator with liquid crystal (hereinafter, referred to as LC-SLM) has a lower speed of response while it can be driven at a low voltage, has a high contrast and can display a half tone. Various types thereof have been already proposed. However, a fundamental arrangement of the optically writing type spatial light modulator has the form of laminated photoconductive layers and a liquid crystal layer. Almost all of optically writing type spatial light modulators have been of a reflective type but not transmissive type.
As a transmissive type LC-SLM, having such a threshold characteristic of a spatial light modulator 1003 as shown in FIG. 10, in which a photoconductive layer 1001 made of Bi.sub.12 SiO.sub.20 (hereinafter, referred to as BSO) or Bi.sub.12 GeO.sub.20 and a liquid crystal layer 1002 are laminated, has been proposed (Kuniji Takizawa et al, Preliminary Lecture Brief of The 50th Applied Physics Society Scientific Lecture Meeting, Autumn in 1989, 28p-ZD-6, 30p-ZD-7 and 28p-ZD-8).
In accordance with the lamination type spatial light modulator with the liquid crystal as shown in FIG. 10, a dark electric impedence of the photoconductive layer upon energization must be greater than an electric impedance of the liquid crystal layer so that a voltage applied to the liquid crystal is not much higher when light is shut off. However, the liquid crystal has a low capacitance and a great resistivity because it is an organic matter. Therefore, the electric impedence of the liquid crystal is generally high. Thus, the thickness of the photoconductive layer must be correspondingly increased so as to decrease the capacitance of the photoconductive layer and to increase the electric resistance thereof. Consequently, the thickness of a photoconductive layer made of a photoconductive material, e.g., CdS, CdSe or a-Si:H which are generally used is inevitably increased, resulting in insufficient light transmission through the photoconductive layer. Because of this fact, almost all of the optically-writing type spatial light to modulators using liquid crystal have been of the reflective type.
The optically-writing type transmissive spatial light modulator is more preferable than the optically-writing type reflective spatial light modulator because the former requires only a single light source so as to have a more simplified optical system. In particular, one of the important problems is how to simply form a hierarchical network configuration for execution of neurocomputing. Although the example proposed by Ishikawa et al has a network with a simplified two-layer structure, an optical system of this network is very complicated. This is because that this optical system employed a microchannel spatial light modulator of a reflective type. Since the hierarchical structure of the hierarchical neural network having at least three layers is required for realizing a practical function, the neural network must comprise a transmissive spatial light modulator. Otherwise, the hardware of the neural network would be too complicated to realize the neural network.
In addition, one of fundamental operations of the neural network takes the total value of multiple inputs and subjects the total value to a threshold operation. The thresholds must be controlled in order to accelerate convergence of learning and to improve operation in an ambiguous or fuzzy process. However, an optical neurocomputer of which the neural network is in the form of hardware is currently executing the threshold operation by means of an electronic circuit or electronic computer because no spatial light modulators having such a control function are present. Consequently, executing the threshold operation entails a photoelectric conversion so that the formation of the hierachical network is very difficult.
The prior-art example of FIG. 10 has realized a transmissive spatial light modulator including the photoconductive layer 1001 made of BSO essentially transparent to visible radiation having a threshold function. However, the prior-art example of FIG. 10 lacks the function of threshold operation by collecting several inputs given by the fundamental operations of the neural network so as to execute the threshold operation. In addition, since the specific dielectric constant of BSO is about 20 times as high as that of a liquid crystal, the thickness of a BSO layer becomes thick as 2 mm. A writing-light intensity for driving a liquid crystal is higher then 1 mW/cm.sup.2, and the operating voltage of the transmissive spatial light modulator of FIG. 10 must be increased to 10 V or higher.